@marcfaaborg: AFAKI, the iOS version of iBooks has limited SMIL support. For example, it won't play the
perfectly valid epub3 audio books by ReadBeyond.
If you're able to distribute your book without DRM, you might be able to use the free iOS/Android
Menestrello apps, which have much better SMIL support than iBooks.
(BTW, the developer,
Alberto Pettarin, is an MR member.)